Re: [PATCH 5/9] fs: remove various compat readv/writev helpers

Christoph Hellwig
2020-09-23T17:46:22Z

On Wed, Sep 23, 2020 at 06:05:27PM +0100, Al Viro wrote:
> On Wed, Sep 23, 2020 at 05:38:31PM +0100, Al Viro wrote:
> > On Wed, Sep 23, 2020 at 03:59:01PM +0100, Al Viro wrote:
> >
> > > > That's a very good question. But it does not just compile but actually
> > > > works. Probably because all the syscall wrappers mean that we don't
> > > > actually generate the normal names. I just tried this:
> > > >
> > > > --- a/include/linux/syscalls.h
> > > > +++ b/include/linux/syscalls.h
> > > > @@ -468,7 +468,7 @@ asmlinkage long sys_lseek(unsigned int fd, off_t offset,
> > > > asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);
> > > > asmlinkage long sys_write(unsigned int fd, const char __user *buf,
> > > > size_t count);
> > > > -asmlinkage long sys_readv(unsigned long fd,
> > > > +asmlinkage long sys_readv(void *fd,
> > > >
> > > > for fun, and the compiler doesn't care either..
> > >
> > > Try to build it for sparc or ppc...
> >
> > FWIW, declarations in syscalls.h used to serve 4 purposes:
> > 1) syscall table initializers needed symbols declared
> > 2) direct calls needed the same
> > 3) catching mismatches between the declarations and definitions
> > 4) centralized list of all syscalls
> >
> > (2) has been (thankfully) reduced for some time; in any case, ksys_... is
> > used for the remaining ones.
> >
> > (1) and (3) are served by syscalls.h in architectures other than x86, arm64
> > and s390. On those 3 (1) is done otherwise (near the syscall table initializer)
> > and (3) is not done at all.
> >
> > I wonder if we should do something like
> >
> > SYSCALL_DECLARE3(readv, unsigned long, fd, const struct iovec __user *, vec,
> > unsigned long, vlen);
> > in syscalls.h instead, and not under that ifdef.
> >
> > Let it expand to declaration of sys_...() in generic case and, on x86, into
> > __do_sys_...() and __ia32_sys_...()/__x64_sys_...(), with types matching
> > what SYSCALL_DEFINE ends up using.
> >
> > Similar macro would cover compat_sys_...() declarations. That would
> > restore mismatch checking for x86 and friends. AFAICS, the cost wouldn't
> > be terribly high - cpp would have more to chew through in syscalls.h,
> > but it shouldn't be all that costly. Famous last words, of course...
> >
> > Does anybody see fundamental problems with that?
>
> Just to make it clear - I do not propose to fold that into this series;
> there we just need to keep those declarations in sync with fs/read_write.c

Agreed. The above idea generally sounds sane to me.